MEMS based acoustic array

CROSS-REFERENCE TO RELATED PATENT APPLICATION

0001This application is a continuation-in-part and claims the benefit, pursuant to 35 U.S.C. § 119(e), of provisional U.S. patent application Ser. No. 60/253,525, filed Nov. 28, 2000, entitled “MEMS BASED ACOUSTIC ARRAY,” which is incorporated herein in its entirety by reference.

0002This invention was made partially with Government support under Grant No. NAG-1-2133 awarded by NASA Langley Research Center and the Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

00031. Field of the Invention

0004The present invention relates generally to an acoustic combination responsive to an acoustic wave, and more particularly, to an acoustic combination that can be utilized as a dynamic pressure sensor and a microelectromechanical systems (MEMS)-based acoustic array that utilizes the acoustic combination.

00052. Background

0006As aircraft noise regulations become more stringent, the need for modeling and measuring aircraft noise phenomena becomes more important. In order to intelligently design quieter aircraft, the physical mechanisms of noise generation should be understood and any theoretical or computational noise model should be experimentally validated. One validation method is the comparison of the theoretical and measured acoustic far-field pressures. However, single microphone measurements of aeroacoustic sources in wind tunnels are hampered by poor signal to noise ratios that arise from microphone wind self-noise, tunnel system drive noise, reverberation, and electromagnetic interference. In addition, a single microphone cannot distinguish pressure contributions from different source locations. The need for more precise noise source characterization and localization has driven the development of advanced sound field measurement techniques. In particular, the development and application of directional (phased) microphone arrays have been documented as a means to localize and characterize aeroacoustic sources in the presence of high background noise.

0007Although knowledge of the acoustic field does not uniquely define the source, localization of a source and analyses of the spatial and temporal characteristics of its far-field radiation can provide insight into noise generation mechanisms. Modern acoustic arrays used in wind tunnel studies of airframe noise are typically constructed of moderate numbers (≦100) of instrumentation grade condenser microphones, and range in aperture size from several inches to several feet. Data collection, followed by extensive post-processing has been used to implement various beamforming processes, including conventional beamforming, array shading, shear-layer corrections, adaptive methods, etc. The resulting data files can be over 500 GB in size and require up to an hour of post-processing per data set.

0008Greater numbers of microphones in an array can improve the ability to characterize a sound field. A greater number of microphones enhances the signal to noise ratio of an array, defined as the array gain, given (in dB) by 10*log(M), where M is the number of microphones. In addition, a large number of microphones may be used to extend the frequency range of an array. The spatial resolution of an array is related to the product kD, where k=ω/c is the acoustic wavenumber, ω is the radian frequency, c is the speed of sound, and D is the aperture size. Thus, a larger aperture is needed to improve the spatial resolution of an array, of most concern at low frequencies. In contrast, the intersensor spacing must be kept less than one-half of the smallest wavelength of interest (highest frequency) to avoid spatial aliasing. The feasibility of scaling the current technology to multiple arrays with large numbers (hundreds or thousands) of microphones is limited by the cost per channel (microphone, amplifier, data acquisition), data handling efficiency (acquisition capabilities, signal processing complexity, storage requirements), and array mobility (size, weight, cabling). In addition, experiments performed in large wind tunnels are costly and require extensive setup. Thus, an array system that provides near real-time output would be advantageous.

0009Thus, there is a need to develop a new acoustic array system that, among other applications, can be utilized for aeroacoustic measurement.

SUMMARY OF THE INVENTION

0010In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to a combination responsive to an acoustic wave that can be utilized as a dynamic pressure sensor. For example, the combination in a form of acoustic array can be utilized as a dynamic pressure sensor for aeroacoustic measurements for airframe noise mechanism identification and source localization. In one embodiment of the present invention, the combination has a substrate having a first surface and an opposite second surface, a microphone positioned on the first surface of the substrate and having an input and a first output and a second output, wherein the input receives a biased voltage, and the microphone generates an output signal responsive to the acoustic wave between the first output and the second output. The combination further has an amplifier positioned on the first surface of the substrate and having a first input and a second input and an output, wherein the first input of the amplifier is electrically coupled to the first output of the microphone and the second input of the amplifier is electrically coupled to the second output of the microphone for receiving the output signal from the microphone. The amplifier is spaced from the microphone with a separation smaller than 0.5 mm.

0011The substrate comprises a semiconductor material such as silicon. The microphone can be chosen from a variety of available devices. In one embodiment, the microphone comprises a piezoresistive microphone that has a semiconductor substrate having a cavity, a diaphragm covering the cavity, and at least one piezoresistor positioned on top of the diaphragm, wherein when the cavity receives the acoustic wave, the diaphragm moves in response to cause stress in the piezoresistor, and the piezoresistor generates an electric signal responsive to the acoustic wave. The piezoresistor is electrically coupled to the first and second outputs of the microphone to cause an electrical voltage as output signal from the electric signal responsive to the acoustic wave. The combination may further have at least one resistor electrically coupled to the piezoresistor. In one embodiment, the microphone comprises four piezoresistors forming a Wheatstone bridge. Each piezoresistor can be a single-crystal silicon piezoresistor, wherein the piezoresistor may be silicon-dioxide encapsulated. The diaphragm may be a silicon-nitride membrane.

0012The combination may further include a channel in fluid communication with the cavity and the ambient atmosphere, wherein the channel is positioned on the surface of the semiconductor substrate and terminated at one end with a vent hole in fluid communication with the ambient atmosphere.

0013In another aspect, the present invention relates to a method of assembling a combination responsive to an acoustic wave. In one embodiment, the method includes the steps of providing a substrate having a first surface and an opposite second surface, positioning a microphone on the first surface of the substrate, and positioning an amplifier on the first surface of the substrate spaced from the microphone with a separation smaller than 0.5 mm. The microphone has an input and a first output and a second output, and the amplifier has a first input electrically coupled to the first output of the microphone and a second input electrically coupled to the second output of the microphone and an output. The substrate comprises a silicon layer. The microphone can be a piezoresistive microphone.

0014In a further aspect, the present invention relates to a combination responsive to an acoustic wave. In one embodiment, the combination includes a microphone having an input and a first output and a second output, wherein the input receives a biased voltage, and the microphone generates an output signal responsive to the acoustic wave between the first output and the second output. The combination further includes an amplifier having a first input and a second input and an output, wherein the first input of the amplifier is electrically coupled to the first output of the microphone and the second input of the amplifier is electrically coupled to the second output of the microphone for receiving the output signal from the microphone. Additionally, the combination includes a first high pass filter electrically coupled between the first input of the amplifier and the first output of the microphone, and a second high pass filter electrically coupled between the second input of the amplifier and the second output of the microphone, wherein the output signal from the microphone has a DC component and an AC component, and the first and second high pass filters substantially block the DC component and allow the AC component of the output signal from the microphone to pass, thereby to allow the amplifier to generate a low impedance signal at the output.

0015In one embodiment, the microphone comprises four piezoresistors forming a Wheatstone bridge that has a first arm, an opposing second arm, a third arm, and an opposing fourth arm, the first arm being electrically coupled to the input of the microphone, the opposing second arm being electrically coupled to ground, the third arm being electrically coupled to the first input of the amplifier, and the opposing fourth arm being electrically coupled to the second input of the amplifier. The amplifier is a preamplifier. The first high pass filter comprises a resistor and a capacitor, the capacitor being electrically coupled in series between the third arm of the Wheatstone bridge and the first input of the amplifier, and the resistor being electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. Moreover, the second high pass filter comprises a resistor and a capacitor, the capacitor being electrically coupled in series between the fourth arm of the Wheatstone bridge and the second input of the amplifier, and the resistor being electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. The resistor of the first high pass filter and the resistor of the second high pass filter are jointly coupled to the second arm of the Wheatstone bridge. The resistor of the first high pass filter and the resistor of the second high pass filter are substantially identical to each other. For example, each of the resistor of the first high pass filter and the resistor of the second high pass filter can be a low profile surface mount resistor. The capacitor of the first high pass filter and the capacitor of the second high pass filter are substantially identical to each other. For example, each of the capacitor of the first high pass filter and the capacitor of the second high pass filter is a low profile surface mount capacitor.

0016In yet another aspect, the present invention relates to a combination responsive to an acoustic wave. In one embodiment, the combination includes a semiconductor substrate having a first surface and a second surface, a microphone positioned on the first surface of the semiconductor substrate and having an input and a first output and a second output, wherein the input receives a biased voltage, and the microphone generates an output signal responsive to the acoustic wave between the first output and the second output, an amplifier positioned on the first surface of the semiconductor substrate and having a first input and a second input and an output, wherein the first input of the amplifier is electrically coupled to the first output of the microphone and the second input of the amplifier is electrically coupled to the second output of the microphone for receiving the output signal from the microphone, a first capacitor and a first resistor forming a first RC pair positioned on the first surface of the semiconductor substrate and being electrically coupled between the first input of the amplifier and the first output of the microphone, and a second capacitor and a second resistor forming a second RC pair positioned on the first surface of the semiconductor substrate and being electrically coupled between the second input of the amplifier and the second output of the microphone. The output signal from the microphone has a DC component and an AC component, and the first and second RC pairs substantially block the DC component and allow the AC component of the output signal from the microphone to pass, thereby to allow the amplifier to generate a low impedance signal at the output, and wherein the amplifier is spaced from the microphone with a separation smaller than 0.5 mm.

0017In one embodiment, the microphone comprises four piezoresistors forming a Wheatstone bridge that has a first arm, an opposing second arm, a third arm, and an opposing fourth arm, the first arm being electrically coupled to the input of the microphone, the opposing second arm being electrically coupled to ground, the third arm being electrically coupled to the first input of the amplifier, and the opposing fourth arm being electrically coupled to the second input of the amplifier. The first capacitor is electrically coupled in series between the third arm of the Wheatstone bridge and the first input of the amplifier, and the first resistor is electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. Additionally, the second capacitor is electrically coupled in series between the fourth arm of the Wheatstone bridge and the second input of the amplifier, and the second resistor is electrically coupled in parallel between the second arm of the Wheatstone bridge and the capacitor. The first resistor and the second resistor are jointly coupled to the second arm of the Wheatstone bridge. The semiconductor substrate comprises a silicon layer, which can be passivated with silicon dioxide. The combination may further comprise a plurality of metal bond pads for receiving components positioned on the silicon layer, and a plurality of conductive traces for interconnecting corresponding metal bond pads.

0018The combination may further comprise a package positioned underneath the semiconductor substrate. In one embodiment, the package has a body for supporting the semiconductor substrate, and a plurality of pins, each pin being conductive and penetrating through the body. The body is electrically coupled to ground for electromagnetic shielding. The combination can additionally have a plurality of metal wire bonds, wherein each metal wire bond may make an additional electrical coupling to the metal bond pads and the pins. Each metal wire bond may be a gold wire bond.

0019In another embodiment, the combination may further have a package positioned underneath the semiconductor substrate. The package has a body for supporting the semiconductor substrate, and a peripheral portion extending away from the package body. The body is electrically coupled to ground for electromagnetic shielding. The combination further has a lid positioned above the semiconductor substrate. The lid includes a body, and a peripheral portion extending away from the body. The lid body is complementarily sized such that when the lid is positioned over the semiconductor substrate, the peripheral portion of the lid matches to and supported by the peripheral portion of the package, and the body covers the semiconductor substrate and components positioned on the semiconductor substrate. The lid may be made from a material that is substantially transparent to the acoustic wave. Additionally, the lid is electrically coupled to ground for electromagnetic shielding.

0020In another aspect, the present invention relates to a printed circuit board array responsive to an acoustic wave. In one embodiment, the printed circuit board array includes a printed circuit board having a first surface and an opposing second surface, a plurality of sockets distributed over the first surface of the printed circuit board, a laser diode positioned on the first surface of the printed circuit board for aiming the printed circuit board array toward to the acoustic wave, and a plurality of microphone packages, wherein each microphone package is complimentarily sized and received in a corresponding socket and contains at least one microphone responsive to the acoustic wave.

0021The printed circuit board array may further include at least one SMB-type coaxial cable connector positioned on the second surface of the printed circuit board. An SMB-type coaxial cable connector is a type of connector usually used for coaxial cable connections for RF (radio frequency) applications, as known to people skilled in the art. Moreover, the printed circuit board array has at least one support layer positioned above the first surface of the printed circuit board, and at least one support layer positioned below the second surface of the printed circuit board. Each of the support layers may comprise a garolite stiffening layer, which may be bonded to the printed circuit board.

0022In one embodiment, the printed circuit board comprises a double-sided copper clad. The printed circuit board has a center, and the plurality of microphone packages are distributed over the first surface of the printed circuit board along a plurality of concentric rings, each ring having a radius measured from the center of the printed circuit board different from the radius of any other ring. An equal or unequal number of the microphone packages are distributed over each ring, wherein each microphone package has at least one microphone responsive to the acoustic wave and at least one amplifier coupled to and spaced from the microphone with a separation smaller than 0.5 mm.

0023These and other aspects will become apparent from the following description of various embodiments taken in conjunction with the following drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

0024The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and the corresponding experimental results and together with the description, serve to explain the principals of the invention.

0025<figref idref="DRAWINGS">FIG. 1</figref> schematically illustrates the principle of an acoustic array that can be utilized to practice the present invention.

0026<figref idref="DRAWINGS">FIG. 2</figref> shows an acoustic combination in the form of a standard integrated circuit according to one embodiment of the present invention.

0027<figref idref="DRAWINGS">FIG. 3A</figref> shows a cross-sectional view of a microphone that can be utilized in the acoustic combination as shown in <figref idref="DRAWINGS">FIG. 2</figref>.

0028<figref idref="DRAWINGS">FIG. 3B</figref> shows a scanning-electron microscope (SEM) photograph of the microphone as shown in <figref idref="DRAWINGS">FIG. 3A</figref>.

0029<figref idref="DRAWINGS">FIG. 4</figref> shows a circuit schematic of the acoustic combination as shown in <figref idref="DRAWINGS">FIG. 2</figref> in one embodiment of the present invention.

0030<figref idref="DRAWINGS">FIG. 5A</figref> shows an exploded schematic of an acoustic combination in the form of a hybrid microphone-amplifier packaging scheme according to one embodiment of the present invention.

0031<figref idref="DRAWINGS">FIG. 5B</figref> partially shows a perspective schematic of an acoustic combination in the form of a hybrid microphone-amplifier packaging scheme according to one embodiment of the present invention as shown in <figref idref="DRAWINGS">FIG. 5A</figref>, where the components are wire-bonded.

0032<figref idref="DRAWINGS">FIG. 6</figref> shows a perspective schematic of an acoustic combination in the form of a printed circuit board array according to one embodiment of the present invention.

0033<figref idref="DRAWINGS">FIG. 7A</figref> shows a partial plot of array layout that can be utilized for the acoustic combination in the form of a printed circuit board array as shown in <figref idref="DRAWINGS">FIG. 6</figref> according to one embodiment of the present invention.

0034<figref idref="DRAWINGS">FIG. 7B</figref> shows a cross-sectional view of the partial array layout according to <figref idref="DRAWINGS">FIG. 7A</figref> that can be utilized for the acoustic combination in the form of a printed circuit board array as shown in <figref idref="DRAWINGS">FIG. 6</figref> according to one embodiment of the present invention.

0035<figref idref="DRAWINGS">FIG. 8</figref> shows schematically an application of the acoustic combination in the form of a printed circuit board array according to one embodiment of the present invention as shown in <figref idref="DRAWINGS">FIG. 6</figref>.

0036<figref idref="DRAWINGS">FIG. 9</figref> shows magnitude (top portion) and phase response (bottom portion) of a 16 hybrid packages at about 110 dB SPL (re 20 μPa) according to one embodiment of the present invention, respectively.

0037<figref idref="DRAWINGS">FIG. 10</figref> shows relative magnitude (top portion) and phase response (bottom portion) of a 16 hybrid packages according to one embodiment of the present invention, respectively.

0038<figref idref="DRAWINGS">FIG. 11</figref> shows measured array pressure response (left portion) and contour plot (right portion) at 2 kHz for a 48 in.×48 in. scan plane at 36 in. for a hybrid packages according to one embodiment of the present invention, respectively.

0039<figref idref="DRAWINGS">FIG. 12</figref> shows measured array pressure response (left portion) and contour plot (right portion) at 6 kHz for a 48 in.×48 in. scan plane at 36 in. for a hybrid packages according to one embodiment of the present invention, respectively.

0040<figref idref="DRAWINGS">FIG. 13</figref> shows measured array pressure response (left portion) and contour plot (right portion) at 10 kHz for a 48 in.×48 in. scan plane at 36 in. for a hybrid packages according to one embodiment of the present invention, respectively.

0041<figref idref="DRAWINGS">FIG. 14</figref> shows theoretical and measured 0.5 dB beamwidths vs. frequency for source at 36 in. for a hybrid packages according to one embodiment of the present invention.

0042<figref idref="DRAWINGS">FIG. 15</figref> shows weighted RMS error as a function of frequency for a 48 in.×48 in. scan plane at 36 in. for a hybrid packages according to one embodiment of the present invention.

0043<figref idref="DRAWINGS">FIG. 16</figref> shows measured array pressure response (left portion) and contour plot (right portion) at 6 kHz for free field pressure of 44.7 dB SPL (noise floor) for a hybrid packages according to one embodiment of the present invention, respectively.

0044<figref idref="DRAWINGS">FIG. 17</figref> shows measured array pressure response (left portion) and contour plot (right portion) at 6 kHz for free field pressure of 54.7 dB SPL (noise floor) for a hybrid packages according to one embodiment of the present invention, respectively.

0045<figref idref="DRAWINGS">FIG. 18</figref> shows measured array pressure response (left portion) and contour plot (right portion) at 6 kHz for free field pressure of 68.1 dB SPL (noise floor) for a hybrid packages according to one embodiment of the present invention, respectively.

0046<figref idref="DRAWINGS">FIG. 19</figref> shows average array microphone pressure an d peak array response pressure vs. free-field pressure at 6 kHz for a hybrid packages according to one embodiment of the present invention.

0047<figref idref="DRAWINGS">FIG. 20</figref> shows weighted RMS error as a function of free-field sound pressure for a 6 kHz source located at 36 in. for a hybrid packages according to one embodiment of the present invention.

0048<figref idref="DRAWINGS">FIG. 21</figref> shows measured array pressure response (left portion) and contour plot (right portion) for a 6 kHz source located at (12 in., 0 in., 36 in.) for a hybrid packages according to one embodiment of the present invention, respectively.

0049<figref idref="DRAWINGS">FIG. 22</figref> shows absolute spatial error (in inches) in the peak response of the array for a 6 kHz source for a hybrid packages according to one embodiment of the present invention.

0050<figref idref="DRAWINGS">FIG. 23</figref> shows weighted RMS error at 6 kHz as a function of source x-location for a hybrid packages according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

0051The present invention is more particularly described in the following examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular form “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise.

0052Additionally, as used in the specification and in the claims, an acoustic array is a collection of spatially distributed microphones used to measure an acoustic field. The time signals from each microphone are selectively weighted and delayed through a signal processing technique known as beamforming. Beamforming provides the array with a directional response, which is electronically “steered” in space. As illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, a monopole acoustic source <b>1</b> is radiating spherical waves <b>3</b> into a homogeneous quiescent medium fixed in space, where an acoustic array <b>5</b> is positioned. Each microphone <b>2</b> in the array <b>5</b> senses a slightly different phase-shifted waveform <b>7</b> depending on its distance from the source <b>1</b> due to propagation delay. The array <b>5</b> can be focused on the source <b>1</b> by phase shifting the microphones through phase shifting device <b>4</b> and then summing the output <b>7</b>′ from each microphone at a summation device <b>6</b>, which in turn generates a signal <b>9</b> responsive to the acoustic wave <b>3</b>. This process is commonly referred to as delay-and-sum beamforming. The beamforming technique permits the measurement of noise from predefined regions in space, while providing signal rejection for sources located outside of the acoustic beam.

0053Referring generally to <figref idref="DRAWINGS">FIGS. 2–23</figref>, in accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to a combination responsive to an acoustic wave that can be utilized as a dynamic pressure sensor.

0054Referring first to <figref idref="DRAWINGS">FIGS. 2–4</figref>, in one embodiment of the present invention, a combination <b>100</b> in the form of a standard integrated circuit has a substrate <b>10</b> having a first surface <b>12</b> and an opposite second surface <b>14</b>, a microphone <b>20</b> positioned on the first surface <b>12</b> of the substrate <b>12</b>. In the embodiment as shown in <figref idref="DRAWINGS">FIG. 4</figref>, the microphone <b>420</b> having an input <b>422</b> and a first output <b>424</b> and a second output <b>426</b>, wherein the input <b>422</b> receives a biased voltage V_b, and the microphone <b>420</b> generates an output signal responsive to the acoustic wave (not shown) between the first output <b>424</b> and the second output <b>426</b>.

0055The combination <b>100</b> further has an amplifier <b>30</b> positioned on the first surface <b>12</b> of the substrate <b>10</b>. In the embodiment as shown in <figref idref="DRAWINGS">FIG. 4</figref>, the amplifier <b>430</b> has a first input <b>432</b> and a second input <b>434</b> and an output <b>436</b>, wherein the first input <b>432</b> of the amplifier is electrically coupled to the first output <b>424</b> of the microphone <b>420</b> and the second input <b>434</b> of the amplifier <b>430</b> is electrically coupled to the second output <b>426</b> of the microphone <b>420</b> for receiving the output signal from the microphone <b>420</b>. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the amplifier <b>30</b> is spaced from the microphone <b>20</b> with a separation s that is smaller than 0.5 mm. This hybrid approach of a microphone and an amplifier in a combination permits optimization in design and fabrication of both the microphone and the associated circuitry in a standard IC package. Additionally, the close proximity of the amplifier <b>30</b> to the microphone <b>20</b> can reduce electromagnetic interference normally associated with cabling.

0056The substrate <b>10</b> comprises a semiconductor material such as silicon. The microphone <b>20</b> can be chosen from a variety of available microphones. All microphones are based on the electronic measurement of a pressure-induced structural deflection. The microphone is an electro-mechanical-acoustic transducer that transforms acoustical energy into electrical energy. Most commercial (non-MEMS) microphones employ capacitive sensing schemes. The implementation of a capacitive scheme in solid-state sensors requires on-chip electronics to minimize the effects of parasitic capacitance. The piezoresistive transduction scheme, which includes measuring the strain on the top surface of a deflected diaphragm is less expensive to develop, simpler to fabricate, and potentially more robust than a capacitive device. Furthermore, theoretical studies indicate that piezoresistive sensing schemes can transduce a lower minimum detectable pressure than capacitive schemes for diaphragm edge-lengths of less than 0.5 mm. These factors, along with concerns about environmental stability (e.g., condensation) and ease of fabrication, leads the inventors to select a piezoresistive sensing scheme in one embodiment of the present invention.

0057In one embodiment, the microphone can be a piezoresistive microphone <b>320</b> as shown in <figref idref="DRAWINGS">FIGS. 3A and 3B</figref>. The piezoresistive microphone <b>320</b> has a semiconductor substrate <b>310</b> having a cavity <b>322</b>, a diaphragm <b>324</b> covering the cavity <b>322</b>, and at least one piezoresistor <b>326</b> positioned on top of the diaphragm <b>324</b>. In operation, when the cavity <b>322</b> receives the acoustic wave, the diaphragm <b>324</b> moves in response to cause stress in the piezoresistor <b>326</b>, and the piezoresistor <b>326</b> generates an electric signal responsive to the acoustic wave. In addition to active resistors such as piezoresistor <b>326</b>, one or more passive or ordinary resistors <b>328</b> may be electrically coupled to the piezoresistor <b>326</b>. Piezoresistors <b>326</b> can be coupled in various configurations. For example, in one embodiment as shown in <figref idref="DRAWINGS">FIG. 4</figref>, the microphone <b>420</b> has four piezoresistors <b>428</b><i>a</i>–<b>428</b><i>d </i>forming a Wheatstone bridge. Each piezoresistor <b>326</b> can be a single-crystal silicon piezoresistor and may be silicon-dioxide encapsulated to improve stability and reduce noise. In one embodiment, the piezoresistors are encapsulated in a silicon dioxide passivation layer to improve stability and reduce noise. Mobile ions in the passivation layer can drift over time. In addition, the surface of the passivation layer may become hydrated with water moisture that will change the surface potential. Changes in the surface potential and the mobile ion distribution may result in a temporal variation of the electric field at the piezoresistive silicon surface. This in turn may modulate the carrier concentration and change the resistance. Therefore, methods to reduce the mobile ion concentration and control the surface potential and properties are necessary for improved stability. Alternative dielectrics such as silicon nitride and other materials may improve resistance to moisture. The diaphragm <b>324</b> may be a silicon-nitride membrane.

0058The piezoresistive microphone <b>320</b> may further include a channel <b>330</b> in fluid communication with the cavity <b>322</b> and the ambient atmosphere. The channel <b>330</b> is positioned on the surface <b>312</b> of the semiconductor substrate <b>310</b> and terminated at one end with a vent hole <b>332</b> in fluid communication with the ambient atmosphere. The channel <b>330</b> is terminated at another end <b>324</b> in fluid communication with the cavity <b>322</b>.

0059As an example and as shown in <figref idref="DRAWINGS">FIGS. 3A and 3B</figref>, the microphone <b>320</b> can be constructed according to the present invention to include 0.1 μm-thick, silicon-dioxide encapsulated, single-crystal silicon piezoresistors <b>326</b> on top of a 0.15 μm-thick, 210 μm-diameter silicon-nitride membrane <b>324</b> stretched over a 10 μm-deep cavity <b>322</b>. As known for people skilled in the art, while specific values are given here for the thickness, diameter or other physical dimensions of the device as an example, other values can also be used to practice the present invention. Pressure equilibration is achieved by the winding vent channel <b>330</b> that is vented to the surface. This example produces devices with known material properties and strict geometry control (thickness and diameter). The use of dielectrically-isolated, single-crystal silicon piezoresistors offers the following advantages over polycrystalline silicon: the piezoresistive gauge factor of single-crystal silicon is larger than the gauge factor of polycrystalline silicon and monocrystalline silicon does not possess grain boundaries which lead to excessive 1/f noise. Therefore, monocrystalline devices can produce higher-sensitivity, lower-noise microphones.

0060Referring now to <figref idref="DRAWINGS">FIG. 4</figref>, the combination <b>400</b> includes a first high pass filter <b>440</b> electrically coupled between the first input <b>432</b> of the amplifier <b>430</b> and the first output <b>424</b> of the microphone <b>420</b>, and a second high pass filter <b>442</b> electrically coupled between the second input <b>434</b> of the amplifier <b>430</b> and the second output <b>426</b> of the microphone <b>420</b>. In operation, the output signal from the microphone <b>420</b> has a DC component and an AC component, and the first and second high pass filters <b>440</b>,<b>442</b> substantially block the DC component and allow the AC component of the output signal from the microphone <b>420</b> to pass, thereby to allow the amplifier <b>430</b> to generate a low impedance signal at the output <b>436</b>. The first high pass filter <b>440</b> and the second high pass filter <b>442</b> each can be an RC pair circuit, a filter or the like.

0061Still referring to <figref idref="DRAWINGS">FIG. 4</figref>, in this embodiment, the microphone <b>420</b> comprises four piezoresistors <b>428</b><i>a</i>, <b>428</b><i>b</i>, <b>428</b><i>c </i>and <b>428</b><i>d </i>forming a Wheatstone bridge that has a first arm <b>421</b>, an opposing second arm <b>423</b>, a third arm <b>425</b>, and an opposing fourth arm <b>427</b>. The first arm <b>421</b> is electrically coupled to the input <b>422</b> of the microphone <b>420</b>, the opposing second arm <b>423</b> is electrically coupled to ground, the third arm <b>425</b> is electrically coupled to the first input <b>432</b> of the amplifier <b>430</b>, and the opposing fourth arm <b>421</b> is electrically coupled to the second input <b>434</b> of the amplifier <b>430</b>. The amplifier <b>430</b> can be a preamplifier. The first high pass filter <b>440</b> comprises a resistor <b>441</b> and a capacitor <b>443</b>, the capacitor <b>443</b> being electrically coupled in series between the third arm <b>425</b> of the Wheatstone bridge and the first input <b>432</b> of the amplifier <b>430</b>, and the resistor <b>441</b> being electrically coupled in parallel between the second arm <b>423</b> of the Wheatstone bridge and the capacitor <b>443</b>. Moreover, the second high pass filter <b>442</b> comprises a resistor <b>445</b> and a capacitor <b>447</b>, the capacitor <b>447</b> being electrically coupled in series between the fourth arm <b>427</b> of the Wheatstone bridge and the second input <b>434</b> of the amplifier <b>430</b>, and the resistor <b>445</b> being electrically coupled in parallel between the second arm <b>423</b> of the Wheatstone bridge and the capacitor <b>447</b>. The resistor <b>441</b> of the first high pass filter <b>440</b> and the resistor <b>445</b> of the second high pass filter <b>442</b> are jointly coupled to the second arm <b>423</b> of the Wheatstone bridge. In this embodiment, the resistor <b>441</b> of the first high pass filter <b>440</b> and the resistor <b>445</b> of the second high pass filter <b>442</b> are substantially identical to each other. For example, each of them can be a low profile surface mount resistor. Each of them can have different or same physical shapes such as an arc resistor, a tapered resistor or the like. The capacitor <b>443</b> of the first high pass filter <b>440</b> and the capacitor <b>447</b> of the second high pass filter <b>442</b> are substantially identical to each other as well. For example, each of them can be a low profile surface mount capacitor. In this embodiment, the resistor <b>441</b> and the capacitor <b>443</b> form a first RC pair, and the resistor <b>445</b> and the capacitor <b>447</b> form a second RC pair.

0062The semiconductor substrate <b>10</b> comprises a silicon layer, which can be passivated with silicon dioxide. The combination <b>100</b> may further comprise a plurality of conductive leads <b>16</b> for interconnecting corresponding components positioned on the layer <b>10</b>.

0063Referring now to <figref idref="DRAWINGS">FIGS. 5A and 5B</figref>, a combination <b>500</b> responsive to an acoustic wave according to one embodiment of the present invention comprises a package <b>550</b> positioned underneath the semiconductor substrate <b>510</b>. In one embodiment, the package <b>550</b> has a body <b>552</b> for supporting the semiconductor substrate <b>510</b>, and a plurality of pins <b>556</b>, each pin being conductive and penetrating through the body <b>556</b>. The package <b>550</b> further has a peripheral portion <b>554</b> extending away from the package body <b>552</b>. The dimension of the package may be characterized by a diameter d. In one embodiment, the diameter d is chosen to have a value about 0.60 in. The body <b>556</b> is electrically coupled to ground for electromagnetic shielding. The combination <b>500</b> further has a plurality of metal bond pads <b>558</b> for receiving components positioned on the silicon layer <b>510</b>, and a plurality of conductive traces <b>560</b> for interconnecting corresponding metal bond pads <b>558</b>. The combination <b>500</b> can additionally have a plurality of metal wire bonds <b>562</b>, wherein each metal wire bond <b>562</b> may make an additional electrical coupling to the corresponding metal bond pads <b>558</b> and the pins <b>556</b>. Each metal wire bond <b>562</b> may be a gold wire bond.

0064The combination <b>500</b> further has a lid <b>570</b> positioned above the semiconductor substrate <b>510</b>. The lid <b>570</b> includes a body <b>572</b>, and a peripheral portion <b>574</b> extending away from the body <b>572</b>. The lid body <b>572</b> is complementarily sized such that when the lid <b>570</b> is positioned over the semiconductor substrate <b>510</b>, the peripheral portion <b>574</b> of the lid <b>570</b> matches to and supported by the peripheral portion <b>554</b> of the package <b>550</b>, and the body <b>572</b> covers the semiconductor substrate <b>510</b> and components positioned on the semiconductor substrate <b>510</b>. The lid <b>570</b> may be made from a material that is substantially transparent to the acoustic wave. Additionally, the lid <b>570</b> is electrically coupled to ground for electromagnetic shielding.

0065In another aspect, the present invention relates to a printed circuit board array responsive to an acoustic wave. Referring now to <figref idref="DRAWINGS">FIG. 6</figref>, in one embodiment, the printed circuit board array <b>680</b> includes a printed circuit board <b>610</b> having a first surface <b>612</b> and an opposing second surface <b>614</b>, a plurality of sockets <b>616</b> distributed over the first surface <b>612</b> of the printed circuit board <b>610</b>, a laser diode <b>618</b> positioned on the first surface <b>612</b> of the printed circuit board <b>610</b> for aiming the printed circuit board array <b>680</b> toward to the acoustic wave, and a plurality of microphone packages <b>600</b>. Each microphone package <b>600</b> is complimentarily sized and received in a corresponding socket <b>616</b> and contains at least one microphone responsive to the acoustic wave and substantially like the combination <b>100</b>, the combination <b>500</b> or the like of the present invention. Additionally, the printed circuit board array <b>680</b> may have one or more signal processing chips <b>620</b> for on board signal processing. The printed circuit board array <b>680</b> may have a support <b>624</b> to support the printed circuit board <b>610</b>. The support <b>624</b> may be adjustable for height as well orientation.

0066Referring now to <figref idref="DRAWINGS">FIGS. 7A and 7B</figref>, a printed circuit board array <b>780</b> may further include at least one SMB-type coaxial cable connector <b>760</b> positioned on the second surface <b>714</b> of the printed circuit board <b>710</b>. Moreover, the printed circuit board array <b>780</b> has at least one support layer <b>720</b> positioned above the first surface <b>712</b> of the printed circuit board <b>710</b>, and at least one support layer <b>740</b> positioned below the second surface <b>714</b> of the printed circuit board <b>710</b>. Additional support layers <b>720</b>, <b>750</b> may also be utilized. Each of the support layers may comprise a garolite stiffening layer, which may be bonded to the printed circuit board <b>710</b> through, for example, bolts <b>762</b> and corresponding nuts <b>764</b>.

0067The printed circuit board <b>710</b> may comprise a double-sided copper clad. In the embodiment as shown in <figref idref="DRAWINGS">FIGS. 7A and 7B</figref>, the printed circuit board has a center <b>701</b>, and the plurality of microphone packages <b>700</b> are distributed over the first surface <b>712</b> of the printed circuit board <b>710</b> along a plurality of concentric rings <b>703</b>, <b>705</b>. Note that although only a two-ring configuration is shown, configurations have less or more rings can also be utilized to practice the present invention. Each ring has a radius measured from the center <b>700</b> of the printed circuit board <b>710</b> different from the radius of any other ring. For example, ring <b>703</b> has a radius r₁that is different from r₂of ring <b>705</b>. An equal or unequal number of the microphone packages <b>700</b> are distributed over each ring, wherein each microphone package <b>700</b> has at least one microphone responsive to the acoustic wave and at least one amplifier coupled to and spaced from the microphone with a separation smaller than 0.5 mm as discussed above. The printed circuit board <b>710</b> may also have a controller (not shown) for data processing.

0068The printed circuit board array <b>680</b> or <b>780</b> can be utilized for aeroacoustic measurements for airframe noise mechanism identification and source localization. As shown in <figref idref="DRAWINGS">FIG. 8</figref>, an airplane <b>801</b> is radiating spherical waves <b>803</b>, which is received by an acoustic array <b>880</b> of the present invention. Each microphone <b>802</b> in the array <b>880</b> senses a slightly different phase-shifted waveform and generates a signal responsive to the acoustic wave <b>803</b>. The signal is amplified by each corresponding amplifier <b>804</b>. The total signal can be further processed by processing chip(s) <b>820</b>. Laser diode <b>818</b> may aim the acoustic array <b>880</b> more accurately to the airplane <b>801</b>.

0069The invention will be better understood by reference to the following illustrative samples and corresponding testing results, which are constructed and performed according to the present invention.

0000Hybrid Microphone-Amplifier Packages

0070A hybrid microphone-amplifier package scheme combines a micromachined piezoresistive silicon microphone and an Analog Devices AD624 low-noise differential amplifier into a 16-pin, 0.6″ TO-8 semiconductor package. The differential outputs of the microphone Wheatstone bridge are AC-coupled to the inputs of the amplifier via two resistor-capacitor (RC) pairs as shown in <figref idref="DRAWINGS">FIG. 4</figref>, with a cut-on frequency given by, f_c=1/(2πRC) =1.6 Hz. This hybrid package provides a small, self-contained microphone module with an amplified, low-impedance output. The sensor packages are fitted into sockets on a printed circuit board array, permitting external calibration and interchangeability. In this embodiment, related parameters are chosen as R₀≈600 Ω, R=147 kΩ, C=0.68 μF, G=500, V_b=3 V, V_s=±10 V.

0071The construction of the hybrid package including four layers as shown in <figref idref="DRAWINGS">FIGS. 5A and 5B</figref>: the package body, a silicon substrate, the component layer, and a protective lid. A TO-8 “can” package serves as a primary structural element. A silicon substrate is bonded to the can body using conductive silver epoxy. The substrate, passivated with silicon dioxide, provides metal bond pads and interconnecting traces for the devices. The components are bonded to the exposed bond pads of the silicon substrate using conductive silver epoxy. Gold wire bonds are used to make additional electrical connections to the chip bond pads and package pins. A slotted lid provides protection against physical damage while permitting acoustic waves to pass. The can body and lid are connected to the circuit ground for additional electromagnetic shielding.

0000Printed Circuit Board Array

0072An illustrative array of the invention has 16 microphones arranged as partially shown in <figref idref="DRAWINGS">FIGS. 7A and 7B</figref>. The planar layout has four concentric rings with radii 1.80″, 1.94″, 3.60″, and 3.89″, each having four microphones. The array is constructed from a double-sided copper clad PCB that serves as the electrical interface and mechanical structure. The top surface of the PCB contains the 16 microphone packages and a laser diode to permit accurate aiming of the array. The bottom surface contains SMB connectors for the coaxial cabling. Four layers of garolite are milled and through-bolted to the circuit board array to provide additional rigidity.

0000Signal Processing

0073The signal processing involves continuously sampling data from the array, computing the Fast Fourier Transform (FFT) on blocks of data, and using conventional frequency-domain beamforming methods to obtain the array pressure response over a scanned region in space.

0074An Agilent E1432A 16-channel, 16-bit, VXI-based digitizer is used to acquire the signals from the array. The digitizer is interfaced to a host computer (866 MHz Pentium III, 256 MB RAM) via a National Instruments MXI-2 interface bus. The host controls the operation of the digitizer, runs the beamforming algorithms, and displays and saves the results using MATLAB v.6.0. In operation, the digitizer samples all 16 channels with 16-bit resolution at a sampling rate of 25.6 kHz. The Agilent E1432A provides the capability to perform real-time Fast Fourier Transforms (FFT's) on the incoming data, significantly reducing the computational load on the host computer. A Hanning window is used in computing 1024-point FFT's, yielding a frequency resolution of 25 Hz. The digitizer internally compensates for the power lost in the windowing operation by scaling the output by the Hanning window weighting factor of √{square root over (8/3)}. Typically 400 non-overlapping blocks are used, corresponding to 16 sec of time data.

0075The cross-spectral matrices for all 512 bin frequencies are computed in real time for each successive block of FFT data that is transferred to the PC host. The time-averaged cross-spectral matrices are obtained by,

0076<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mrow><msub><mover><mi>R</mi><mo>^</mo></mover><mi>k</mi></msub><mo>=</mo><mrow><mfrac><mn>1</mn><mi>L</mi></mfrac><mo>⁢</mo><mrow><munderover><mo>∑</mo><mrow><mi>l</mi><mo>=</mo><mn>1</mn></mrow><mi>L</mi></munderover><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mrow><msub><mi>Y</mi><mi>kl</mi></msub><mo>⁢</mo><msubsup><mi>Y</mi><mi>kl</mi><mi>′</mi></msubsup></mrow></mrow></mrow></mrow></math></maths><br /> where vector Y_kis a column vector containing the k^thFFT coefficients for all M channels,

0077<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mrow><msub><mi>Y</mi><mi>k</mi></msub><mo>=</mo><mrow><mo>[</mo><mtable><mtr><mtd><msub><mi>Y</mi><msub><mn>1</mn><mi>k</mi></msub></msub></mtd></mtr><mtr><mtd><mi>⋮</mi></mtd></mtr><mtr><mtd><msub><mi>Y</mi><msub><mi>M</mi><mi>k</mi></msub></msub></mtd></mtr></mtable><mo>]</mo></mrow></mrow></math></maths><br /> The data is converted to units of pressure squared (Pa²) by dividing by the square of the microphone sensitivity. The array power response is obtained using <br /><i>{circumflex over (P)}</i>_k<i>=e′</i>_k<i>{circumflex over (R)}</i>_k<i>e</i>_k<br /> where the quantity e_k, known as the steering vector, contains the weights and phase shifts to be applied to the system and is given by

0078<maths id="MATH-US-00003" num="00003"><math overflow="scroll"><mrow><msub><mi>e</mi><mi>k</mi></msub><mo>=</mo><mrow><mrow><mo>[</mo><mtable><mtr><mtd><mrow><msub><mi>w</mi><mi>i</mi></msub><mo>⁢</mo><msup><mi>ⅇ</mi><mrow><msub><mi>jω</mi><mi>k</mi></msub><mo>⁢</mo><msub><mi>Δ</mi><mi>i</mi></msub></mrow></msup></mrow></mtd></mtr><mtr><mtd><mi>⋮</mi></mtd></mtr><mtr><mtd><mrow><msub><mi>w</mi><mi>M</mi></msub><mo>⁢</mo><msup><mi>ⅇ</mi><mrow><msub><mi>jω</mi><mi>k</mi></msub><mo>⁢</mo><msub><mi>Δ</mi><mi>M</mi></msub></mrow></msup></mrow></mtd></mtr></mtable><mo>]</mo></mrow><mo>.</mo></mrow></mrow></math></maths><br /> The weights and phase shifts are given by

0079<maths id="MATH-US-00004" num="00004"><math overflow="scroll"><mrow><mrow><msub><mi>w</mi><mi>m</mi></msub><mo>=</mo><mfrac><msub><mi>r</mi><mi>m</mi></msub><mi>r</mi></mfrac></mrow><mo>,</mo><mstyle><mtext></mtext></mstyle><mo>⁢</mo><mrow><msub><mi>Δ</mi><mi>m</mi></msub><mo>=</mo><mfrac><mrow><mi>r</mi><mo>-</mo><msub><mi>r</mi><mi>m</mi></msub></mrow><mi>c</mi></mfrac></mrow><mo>,</mo></mrow></math></maths><br /> where r and r_mare the radial distances from the focus location to the array center and the m^thmicrophone, respectively, and c is the speed of sound. The array power response is divided by the number of microphones squared (M²) to normalize the array output to that of a single microphone. The pressure response is given by the square root of this result. For most of the measurements made in calibrating the array, a 48 in.×48 in. grid of regularly spaced (1 in. increments) focal locations are used in a scan plane parallel to the array face at a distance of 36 in. The pressure response computation time for a single frequency bin is under 4 sec. The time-averaged cross-spectral matrices for all 512 frequency bins are stored to disk, resulting in file size of 2 MB. <br /> Experimental Results

0080The experimental methodologies and results for calibrations of the hybrid package and array system according to one embodiment of the present invention are presented. The frequency responses of the hybrid microphone packages are obtained using a plane wave tube acoustic calibrator. Characterization of the array system is performed using an acoustic point source in an anechoic chamber.

0000Hybrid Package Calibration

0081Each hybrid microphone package is operated individually for amplitude and phase using a 1″×1″ square cross-section, normal incidence plane wave tube, designed to support plane waves up to 6.7 kHz. The microphone package and a ⅛-inch Bruel and Kjaer (B&amp;K) 4138 reference microphone are flush mounted at the terminating end of the tube and subjected to normally incident plane waves. The frequency response of the hybrid package is determined with respect to the B&amp;K microphone. The frequency response calibrations for the sixteen hybrid packages are shown in <figref idref="DRAWINGS">FIG. 9</figref>. The average measured sensitivity of each hybrid package over the frequency range of 1 kHz–6.5 kHz is shown to vary from 780 μV/Pa to 855 μV/Pa. The mean sensitivity for all sensors over the frequency range tested is 831 μV/Pa. The linear trend in the phase response could be a phase variation but may also be attributed to a mounting error in the PWT calibration. An axial offset in the position in the PWT of the reference microphone and test microphone will result in a linear bias in the phase measurement. An offset of only 1 mm would result in a phase bias error of 6.25 degrees at 6 kHz.

0082For the purposes of array signal processing, the relative magnitudes or phases between sensor packages are important. The frequency response data is reformatted and shown in <figref idref="DRAWINGS">FIG. 10</figref> as a relative magnitude and phase with respect to microphone #1. The magnitude responses of all microphones are shown to match within ±0.6 dB, and the phase responses are matched within ±1 degree over the frequency range tested. Mosher states that phase matching within ±10 degrees is sufficient for obtaining reasonable results without the need for phase corrections. Therefore, the hybrid microphone packages are considered acceptable for use in the array and all of the array calibrations are presented from uncorrected, raw measurements.

0000Array Calibration

0083The array response to an acoustic point source was measured in an anechoic chamber, having a 100 Hz cutoff frequency, as a function of source frequency, source amplitude, and source location. Each measured response was compared to the theoretical response (array pattern) given by,

0084<maths id="MATH-US-00005" num="00005"><math overflow="scroll"><mrow><mrow><mrow><mi>W</mi><mo>⁢</mo><mrow><mo>(</mo><mrow><mi>ω</mi><mo>,</mo><mover><mi>x</mi><mo>→</mo></mover><mo>,</mo><msup><mover><mi>x</mi><mo>→</mo></mover><mi>′</mi></msup></mrow><mo>)</mo></mrow></mrow><mo>=</mo><mrow><munderover><mo>∑</mo><mrow><mi>m</mi><mo>=</mo><mn>1</mn></mrow><mi>M</mi></munderover><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mrow><mfrac><msub><mi>r</mi><mi>m</mi></msub><mi>r</mi></mfrac><mo>⁢</mo><mfrac><msup><mi>r</mi><mi>′</mi></msup><msubsup><mi>r</mi><mi>m</mi><mi>′</mi></msubsup></mfrac><mo>⁢</mo><mi>exp</mi><mo>⁢</mo><mrow><mo>{</mo><mrow><mi>jω</mi><mo>⁡</mo><mrow><mo>[</mo><mfrac><mrow><mrow><mo>(</mo><mrow><msup><mi>r</mi><mi>′</mi></msup><mo>-</mo><mi>r</mi></mrow><mo>)</mo></mrow><mo>-</mo><mrow><mo>(</mo><mrow><msubsup><mi>r</mi><mi>m</mi><mi>′</mi></msubsup><mo>-</mo><msub><mi>r</mi><mi>m</mi></msub></mrow><mo>)</mo></mrow></mrow><mi>c</mi></mfrac><mo>]</mo></mrow></mrow><mo>}</mo></mrow></mrow></mrow></mrow><mo>,</mo></mrow></math></maths><br /> where the “primed” terms represent the radial distances from the actual source (located at {overscore (x)}′) and the “unprimed” terms represent the radial distances from the focus location (located at {overscore (x)}). The acoustic point source, provided by NASA Langley, consists of a JBL 2426J-compression driver mated to a 21 in. long, 0.75 in. inner diameter metal pipe. The pressure field generated by the device is modeled as a piston at the end of a pipe and performs suitably as a point monopole for frequencies below 11.5 kHz.

0085Several metrics were used to quantify the differences between the measured and theoretical responses. For this analysis, the measured array response is normalized by its peak value for direct comparison to the normalized array pattern. The first metric is comparison of the beamwidths of the mainlobe at 0.5 dB, 1 dB, 3 dB, 6 dB, and 9 dB down from the peak values. Because the mainlobe may not have cylindrical symmetry, an equivalent beamwidth is used. It is obtained by computing the diameter of a circle having the same area as enclosed by the respective contour curve. The second metric is to compute a weighted root mean squared (RMS) error for the measured response. It provides an estimate of the total relative error over K scan locations and is expressed,

0086<maths id="MATH-US-00006" num="00006"><math overflow="scroll"><mrow><mrow><msub><mi>weighted_error</mi><mi>RMS</mi></msub><mo>=</mo><msqrt><mfrac><mrow><munderover><mo>∑</mo><mrow><mi>k</mi><mo>=</mo><mn>1</mn></mrow><mi>K</mi></munderover><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><msup><mrow><msub><mi>M</mi><mi>k</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mrow><msub><mi>T</mi><mi>k</mi></msub><mo>-</mo><msub><mi>M</mi><mi>k</mi></msub></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msup></mrow><mi>K</mi></mfrac></msqrt></mrow><mo>,</mo></mrow></math></maths><br /> where T_kand M_krepresent the normalized theoretical and measured responses at the k^thscan location, respectively. The error is weighted by the measured response M_kto account for the relative effect the errors would have in the measured response. A third metric is to compare the location of the measured peak response to the actual source location. <br /> Array Response vs. Frequency

0087The array response was first examined as a function of frequency for a point source positioned at a distance of 36 in. on the z-axis of the array. Discrete tones were used to achieve an average sound pressure level of approximately 100 dB SPL at the array microphones. The measured array pressure responses are shown at 2 kHz, 6 kHz, and 10 kHz in <figref idref="DRAWINGS">FIGS. 11–13</figref>.

0088The equivalent 3 dB mainlobe beamwidth is shown in <figref idref="DRAWINGS">FIG. 14</figref> for frequencies from 1 kHz to 10 kHz. This data is representative of the results obtained for the equivalent 0.5, 1, 6, and 9 dB beamwidths. The measured beamwidths are shown to closely match the theoretical values for frequencies of 3 kHz and higher. Verification of the mainlobe beamwidth is important from a spatial resolution standpoint, but does not address the remainder of the response, particularly the effect of sidelobes. A measure of the total error is given by the weighted RMS errors as shown in <figref idref="DRAWINGS">FIG. 15</figref>. The error remains below 5% up to 8 kHz before increasing to a maximum of 9% at 10 kHz. Diffraction at the end of a cylinder becomes significant for values of kd larger than 1 or 2, where k is the acoustic wavenumber and d is the radius of the disk. It is noted that for the 0.6 in. package, kd=2 at approximately 7.2 kHz, and this may explain the increase in error above that frequency.

0000Array Response vs. Sound Pressure

0089One benefit of an acoustic array is its improvement in the signal to noise ratio of the measured signal, referred to as the array gain. The use of multiple microphones enables the measurement of source signals that are below the noise floor of any one particular microphone. Thus, an important characteristic of an array is its performance as a function of incident sound pressure, or equivalently the signal to noise ratio of the microphones. For this experiment, the point source is fixed on the boresight of the array at a distance of 36 in. and the array response to a 6 kHz tone is measured at various sound pressure levels. As a reference, the array was removed and a single B&amp;K 4138 microphone was used to measure the free-field sound pressure level at the location of the array origin for several sinusoidal voltage amplitudes supplied to the speaker. The array was then reinstalled and the responses were measured using the calibrated speaker voltage inputs.

0090<figref idref="DRAWINGS">FIGS. 16–18</figref> show the array response at several increasing source amplitudes, respectively. The array response appears to “grow” out of the noise floor. <figref idref="DRAWINGS">FIG. 19</figref> shows the average microphone pressure and the peak array pressure vs. the free-field pressure measured by the single B&amp;K microphone. For a strong, tonal point source, the free field pressure, the average microphone pressure, and the peak array pressure should all be equal. At higher sound pressure levels, the average microphone pressure and peak array pressure converge to within 1 dB, but there is an offset of approximately 5 dB between these values and the free-field value. A 3 dB increase could be explained as a pressure doubling due to a sound hard boundary condition at the face of the array. The additional amplification may be due to additional diffraction effects and/or variations in the sensitivity of the devices. Regardless of the absolute levels, the lower end of the curve illustrates the existence of the array gain. As the incident sound pressure level decreases, the average microphone response asymptotes to 69.1 dB SPL while the peak array response asymptotes to 57.5 dB SPL. Thus, the array can effectively detect a source that is 11.6 dB SPL below the noise floor of the individual microphones. The asymptotic values represent the minimum detectable signals for a 25 Hz bin at 6 kHz. The estimated noise floors for a 1 Hz bin at 6 kHz are 55.1 dB SPL for the hybrid packages and 43.5 dB SPL for the array. Maximum sound pressure levels of at least 160 dB SPL are achievable. <figref idref="DRAWINGS">FIG. 20</figref> shows the weighted RMS error as a function of the free field pressure. The curve shows the error decreasing as the incident sound pressure converging to within 1% of its final value at a free-field pressure of 74 dB SPL.

0000Array Response vs. Location

0091Perhaps one of the most useful aspects of a directional array is its capability for source localization. As a one-dimensional verification, a 6 kHz source at a distance of 36 in. is translated in the x-direction in 3 in. increments to a distance of 24 in., and the performance of the array was examined. Ideally, the array response should be calibrated over a broad range of locations in space. <figref idref="DRAWINGS">FIG. 21</figref> shows an example response for the source 12 in. along the x-axis. <figref idref="DRAWINGS">FIG. 22</figref> shows the weighted RMS error for the array response as a function of the x-location. The error remains constant at approximately 2.5% over the range tested.

0092Of greater importance is the ability of the array to accurately locate a source in space. <figref idref="DRAWINGS">FIG. 23</figref> shows the absolute spatial error of the peak array response plotted vs. the x-location of the source. It should be noted that a finer mesh using a grid of 0.1 in. in the array response was used to obtain the measures of spatial error. The error is seen to randomly fluctuate as a function of position, with a mean value of 0.3 in. These values seem reasonable considering the accuracy in distance measurements in setting up the experiment was ±0.25 in. Thus, the performance at 6 kHz appears to be independent of the source location. A novel directional acoustic array has been presented using MEMS sensor technologies, a hybrid sensor packaging scheme, printed circuit board construction technique, and modem signal processing system to produce a high-speed, cost-effective, modular, array measurement system. In addition to reducing the cost, the use of a printed circuit board as the array structure allows for the potential integration of the signal conditioning, data acquisition, and/or signal processing hardware. The estimated total cost of the 16-channel array, excluding labor and the cost of the data acquisition and signal processing system, is approximately $1140, or $71.25 per channel. This is less than a single ⅛-inch Bruel &amp; Kjaer type 4138 microphone with preamplifier and power supply ($3365), commonly used in conventional microphone arrays. The hybrid packages can be interchanged between low-cost printed circuit boards of various geometries, further reducing the costs. The use of high-speed data acquisition and digital signal processors has enabled near real-time computation of the time-averaged cross-spectral matrices. This provides the user with almost instant access to array response results and eliminates the need to save large amounts of time-series data. The resulting time savings can reduce the experimental costs, particularly for large wind tunnel studies.

0093The results from calibrations of the hybrid package and array confirm and verify the functionality of the system. From plane wave tube calibrations, the hybrid microphone packages show an average sensitivity of 831 μV/Pa with matched magnitude (±0.6 dB) and phase (±1 degree) responses. From tests conducted in an anechoic chamber, the array shows accurate source localization capabilities of ±0.3 in. It has a minimum detectable signal of 47.8 dB SPL for a 1 Hz bin at 6 kHz and a maximum input of 160 dB SPL. The usable frequency range is limited to 3 kHz to 8 kHz. A larger number of sensors can broaden the frequency range by increasing the overall array size while maintaining small intersensor spacings. The array noise floor was experimentally verified to be 11.6 dB below the noise floor of the individual microphones, as predicted by the theoretical array gain.

0094For this array system, an extensive calibration is needed to confirm determine the response over a larger parameter space. The calibration should include in situ calibrations of the microphones and a complete analysis of the directivity and accuracy of the array over a broad frequency range. An array, similar in construction, including more sensors could be constructed to examine the response over a larger frequency range. Additional efforts could be aimed at reducing the size and increasing the physical robustness of the hybrid packages and improving the construction techniques used for the array.

0095The invention has been described herein in considerable detail, in order to comply with the Patent Statutes and to provide those skilled in the art with information needed to apply the novel principles, and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modification, both as to equipment details and operating procedures can be effected without departing from the scope of the invention itself. Further, it should be understood that, although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims.